This invention relates to processes for producing sorbitan fatty acid esters.
It is known that sorbitan fatty acid esters can be produced by direct, base-catalyzed reaction of sorbitol with a fatty acid at elevated temperatures. Such process is used commercially and is disclosed, for example, in U.S. Pat. No. 2,322,820 to Brown. Example 3 of this patent describes such reaction at 260.degree. C. Brown also teaches that products containing both hexitan fatty acid esters and hexide esters can be formed in the presence of an acid catalyst, or with no catalyst, stating that acid catalysts tend to produce esters of hexides, while alkaline catalysts tend to produce esters of hexitans. Brown teaches that preferably the hexahydric alcohol and the fatty acid ester are mixed and reacted in the presence of each other from the beginning, although a disclosed alternative is to first treat the hexahydric alcohol to form an inner ether (hexitan or hexide) and thereafter add the fatty acid for esterification.
U.S. Pat. No. 2,322,821 to Brown describes the preparation of hexide esters, either by direct reaction of a hexitol (sorbitol or mannitol) with a fatty acid in the presence of an acid catalyst, or by esterification of a hexide by a reaction with an acid halide (e.g. lauroyl chloride) in a medium made basic with pyridine.
A disadvantage of base catalyzed direct reaction of sorbitol with a fatty acid is that the product is usually highly colored. Treatment with a bleaching agent, such as hydrogen peroxide, sodium hypophosphite, phosphorous acid, or sodium phosphite, is normally required in order to produce a product having commercially acceptable color.
Japanese Patent Publication No. 15246 of 1974 (published Apr. 13, 1974) discloses a process for preparing a sorbitan ester by reaction of sorbitol with a fatty acid, first in the presence of a basic catalyst at 200.degree.-260.degree. C., then in the presence of an acid catalyst at 180.degree.-240.degree. C. According to the publication, esterification takes place mainly during the first portion of the reaction, when a basic catalyst is used, while anhydrization takes place in the presence of the acid catalyst during the second half of the reaction. Patentee claims that sorbitan fatty acid esters produced in his process contain notably less color than those of previous processes such as that described in U.S. Pat. No. 2,322,821. Representative Gardner color values are about 6 or 7; lower color values can be achieved by treating the reaction with a bleaching agent such as hypophosphorous acid.
J. D. Brandner et al., Industrial and Engineering Chemistry, vol. 37, no. 9, pages 809-812 (1945) describes esterification of sorbitol with linseed fatty acids, both with and without catalyst, at 180.degree. or 200.degree. C. Calcium acetate and barium acetate are disclosed as catalysts. The products are primarily sorbitol esters, although some anhydrization takes place.
U.S. Pat. No. 2,390,395 to Soltzberg describes the preparation of monoanhydro sorbitol which is rich in 1,4-sorbitan by anhydrization of sorbitol under reduced pressure at 120.degree.-150.degree. C. in the presence of an acid catalyst.
U.S. Pat. No. 2,387,842 to Soltzberg discloses the preparation of "sorbide" (actually a mixture of isomers) by heating sorbitol solution at reduced pressure (88-95 mm of mercury absolute) in the presence of an acid catalyst (sulfuric acid) until 2 moles of water per mole of sorbitol are removed.
The art presently relies on the use of bleaching agents to reduce the color of the products. There is needed a process which will produce sorbitan esters having lower colors than those now obtained when bleaching agents are not used.